Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device.
Typically, an LED chip includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact is formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
In order to use an LED chip in a circuit, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, an LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wirebonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup 13 may be filled with an encapsulant material 16 containing a wavelength conversion material such as phosphor particles. The entire assembly may then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. The term “phosphor” is used herein to refer to any materials that absorb light at one wavelength and re-emit light at a different wavelength, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Accordingly, the term “phosphor” is used herein to refer to materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphor particles absorb light having low wavelengths and re-emit light having longer wavelengths.
Typically, phosphor particles are randomly distributed within the matrix of encapsulant material. Some or all of the light emitted by the LED chip 12 at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a blue-emitting chip may be encapsulated with an encapsulant matrix including a yellow-emitting phosphor. The combination of blue light (from the chip) with yellow light (from the phosphor) may produce a light that appears white. Some red-emitting phosphor particles may be included in the encapsulant matrix to improve the color rendering properties of the light, i.e. to make the light appear more “warm.” Similarly, a UV-emitting chip may be encapsulated with an encapsulant material including phosphor particles that individually emit red, green and blue light upon excitation by UV light, The resulting light, which is a combination of red, green and blue light, may appear white and may have good color rendering properties.
However, rays of light emitted by the chip at different angles may follow different path lengths through the encapsulant material, which may result in the emission of different levels of light from the phosphor as a function of angle of emission. Because light may be emitted by the chip 12 in different intensities depending on the angle of emission, light emitted by the package 10 may have an uneven color distribution. Particle settling may also affect the color uniformity of the emitted light.
Furthermore, the volume of encapsulant material surrounding the LED chip 12 may tend to increase the effective size of the light source, which may increase the difficulty of designing secondary optics for the package.
Accordingly, some techniques for directly coating LED chips with phosphors have been proposed. For example, a phosphor coating technique is described in US Patent Publication No. 2006/0063289, assigned to the assignee of the present invention. Other techniques, such as electrophoretic deposition, have been proposed.